Inkjet printing fluid

Abstract

An inkjet printing fluid composition including water and a water dispersible polyurethane additive of the general formula of (I)wherein Z is the central portion of a monomer unit that is the polymerization product of a diisocyanate; X1—Y1—X1 represents one or more soft segments wherein Y1 represents the central portion of a unit that is the polymerization product of a diamine or diol prepolymer having a molecular weight of greater than 300 Daltons; W is the central portion of one or more units containing an acid group; X2—Y2—X2 represents one or more hard segments wherein Y2 represents the central portion of a unit that is the polymerization product of a C2-C8 diol or diamine having a molecular weight of less than 250 Daltons; and X1, V and X2 can be the same or different and are an —O— or —N— atom; and further wherein the polyurethane additive has a weight average molecular weight of at least 6,000 Daltons and a sufficient number of acid groups to provide an acid number greater than 35, and the one or more X2—Y2—X2 hard segments are present at from 1 wt % to less than 13 wt % of the polyurethane additive. Also disclosed is a method for printing an inkjet image including I) providing an inkjet printing fluid according to the invention; and II) jetting the inkjet printing fluid in the form of ink drops onto a recording element to form a printed image.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]Reference is made to commonly assigned U.S. application Ser. No. 12 / 872,018, filed Aug. 31, 2010 (Publication No. 2012 / 0050415), directed towards “Recirculating Fluid Printing System and Method,” the disclosure of which is incorporated by reference herein in its entirety.FIELD OF THE INVENTION[0002]The invention relates generally to the field of inkjet printing fluids, including pigmented and clear inks, and in particular to inks which are useful for thermal inkjet printing and continuous inkjet printing. More specifically, the invention relates to pigmented and clear inks for high-speed inkjet printing which result in durable images.BACKGROUND OF THE INVENTION[0003]Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that can be used to control the deposition of ink droplets o...

AI Technical Summary

Benefits of technology

[0016]It is an object of this invention to provide inkjet printing fluid compositions, including pigment-based inks and clear inks, containing an aqueous polyurethane binder that provides abrasion resistance to an image at both short time and long time intervals after printing. It is a further object to provide such printing fluids which can be jetted at high firing frequencies from an inkjet printhead. It is a further object of this invention to provide such printing fluids for CIJ printing systems which enable the ink to be recirculated in a CIJ fluid system for extended periods without significant pressure build up or filter clogging.

Problems solved by technology

However, pigment-based inks have a number of drawbacks.

Pigment-based inks often require a lengthy milling operation to produce particles in the sub-micron range needed for most modern ink applications.

If the pigment particles are too large light scattering can have a detrimental effect on optical density and gloss in the printed image.

A second drawback of pigmented inks is their durability after printing, especially under conditions where abrasive forces have been applied to the printed image.

Pigment-based inks typically reside at the surface of the imaging receiver to which they are printed and this makes the printed images particularly susceptible to abrasive forces.

At short time intervals the binder film formation is incomplete and the resulting pigment cake is particularly susceptible to abrasive forces.

Images printed from an inkjet printer are also susceptible to abrasive forces as the image receiver is advanced through the printer.

In either case, the mechanical abrasion caused by the spur wheel occurs at short time intervals on the order of a few seconds after printing and results in a defect that is objectionable to the eye.

Further, in some cases, the incomplete polymer binder film formation results in a tacky surface that can stick to such surfaces within the printer that transport the printed image.

Pigmented inks for inkjet printing have been formulated with acrylic polymers; however, the acrylic polymers alone are insufficient in providing durable images that resist scratches and other forms of physical abuse.

Although polyurethanes are known for their excellent abrasion resistance, they also have a number of drawbacks.

US H2113H, and US Publication Numbers 2004 / 0130608 and 2004 / 0229976 are particularly difficult to jet from a thermal inkjet printhead at high tiring frequencies.

Clear inks formulated with polyurethanes, however, also suffer from the same short term durability issues as colored inks since they have many components in common with their colored ink counterparts.

In addition, the application of a clear ink increases the total amount of water applied to the receiver and therefore slows down the drying of the imaged area of the prints.

Although the application of clear ink can improve the long term durability, its application can adversely affect the short term durability due to the increased water load on the receiver.

Both pigment and clear inks can be difficult to jet through inkjet printheads having small nozzle diameters especially by the thermal inkjet printing process.

However, this high frequency firing often comes at the cost of variability in the firing velocity, which leads to poor image quality in the final printed image.

In addition, the demands of current thermal inkjet printing require that the nozzles fire for a large number of firings during the life-time of a printer.

Although there have been many recent advances in the art of dye-based inkjet inks, such inks still suffer from deficiencies such as low optical densities on coated glossy paper and poor light-fastness.

When water is used as the carrier, such inks also generally suffer from poor water fastness and poor smear resistance.

It has been discovered, however, that severe filter plugging issues may be encountered when running pigment inks in a CIJ fluid system, requiring frequent change of filters, e.g., over the time period of a few hours vs. a few months for dye-based inks.

The consequence of filter plugging is the loss of fluid pressure and fluid jets, leading to system shutdown.

Further investigation has discovered that the gear pump commonly used in the fluid system to maintain fluid pressure with minimal pulsation can cause agglomeration of pigment particles, leading to filter clogging and system shutdown.

Further, addition of polyurethanes can also substantially increase the viscosity of a printing fluid, and cause printing fluids to become destabilized upon recirculation, which may be especially undesirable for continuous inkjet printing applications.

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